1. Field of the Invention
This invention relates generally to a power supply circuit and more particularly to a power supply circuit of a switching type.
2. Description of the Prior Art
A power supply circuit of a switching system is well known in the art. In a prior art power supply circuit, a commercial AC voltage is supplied through a power plug and a power switch to a rectifier, and then rectified and smoothed as a DC voltage. This DC voltage is supplied to a series connection of the primary winding of a transformer and a switching transistor which transistor is supplied with PWM (pulse width modulated) signal from a PWM modulator and then switched ON and OFF. Thus, an AC voltage is induced on the secondary winding of the transformer and then fed to a rectifier to be rectified as a desired DC voltage which is delivered to an output terminal.
The output voltage terminal is detected by a detecting circuit and the detected output is fed through a coupler for isolation and separation such as a photocoupler to the modulator as a modulation signal. While, a pulse from an oscillator is applied to the modulator as its carrier. Thus, the width of the PWM pulse from the modulator is varied in response to the voltage at the output terminal and hence the DC voltage at the output terminal is stabilized at a constant value.
Under the following assumption that
V.sub.i : output voltage of first rectifier PA0 D: duty ratio of PWM pulse PA0 T.sub.p : period of PWM pulse PA0 R.sub.L : load connected to output terminal PA0 l.sub.1 : inductance of primary winding of transformer,
the output voltage V.sub.o at the output terminal is expressed as follows: ##EQU1##
The maximum output power P.sub.o is expressed as follows: ##EQU2##
Thus, the output voltage V.sub.o can be made of a constant value by varying the duty ratio D of the PWM pulse.
In this case, however, the period T.sub.p is determined by the characteristics of the switching transistor and the inductance l.sub.1 of the primary winding is determined by the voltage V.sub.i and maximum output power P.sub.o. In this case, the energy stored in the primary winding during the period within which the switching transistor is in ON-state is derived from the secondary winding during the period within which the transistor is in OFF-state, so that in general the duty ratio D is limited to 0.5.about.0.7.
Accordingly, with the above prior art power supply circuit, the output voltage V.sub.o can not be sufficiently stabilized for wide variation of loads.
For this reason, in the art a dummy load is connected to the output terminal of the power supply circuit in addition to the real load to reduce a range of an apparent load variation and to stabilize the output voltage V.sub.o. This method, however, causes lowering of efficiency and becomes ineffective for a wide range of load variation.
A prior art disclosure by way of example is found in U.S. Pat. No. 3,701,937.